Determination of total nitrogen content in fresh leaves and leaf powder of Dendrobium orchids using near-infrared spectroscopy

Abstract

Appropriate application of nitrogen (N) fertilizer promotes plant growth, inflorescence yield, and flower quality of orchids. In this study, we used the near-infrared spectroscopy (NIRS) technique to develop a prediction model of the N content in the leaf of Dendrobium orchid, which is an essential indicator for monitoring plant health. The Dendrobium orchid samples were foliar sprayed in rotation between 20N–20P–20K and 30N–10P–10K fertilizers at a frequency of once or twice a week to create a diverse amount of leaf N content. An application of water was used as the control treatment. After nine months of fertilizer treatments, 150 fresh orchid leaf samples, containing various N contents, were scanned using Fourier-transform near-infrared spectroscopy (FT-NIRS). Then the samples were dried and ground to a fine powder and were again scanned. The absorbance spectra were collected at the 12,000–4000 cm−1 (800–2500 nm) region. Total N content was determined by using the combustion method. The result showed a high proficiency in the estimation of N in leaf powder with the correlation coefficient of prediction (Rp), the root mean square error of prediction (RMSEP), and the ratio of standard deviation of reference data of prediction set to standard error of prediction (RPD) being 0.9882, 0.0637% dry weight (DW), and 6.53, respectively. The fresh leaf sample was successfully predicted with Rp, RMSEP, and RPD of 0.8874, 0.207% DW, and 2.13, respectively. Additionally, external validation confirmed the high reliability of using leaf powder since the correlation coefficient of external validation (rp) and RMSEP of the external validation achieved 0.9651 and 0.1405% DW, respectively, while the external validation of fresh leaf should be improved with rp = 0.7438 and RMSEP = 0.2652% DW. The overall results suggested that NIRS can be used for monitoring the N status in orchids, especially in leaf powder, with high accuracy.